Anatomy Basics:The deltoid

The shoulder
I promise you this site won't turn into something only about human anatomy for the artist. I will always include other subjects for those interested in drawing and painting in general. However, I am personally interested in the figure and I feel that understanding the anatomy to some proficiency will only improve one's ability to draw the figure. With that in mind, I thought this would be a good area to explore for awhile.

Today, we are going to take a look at the shoulder muscle, to understand the basics of its form and function. Let's take a look.



The deltoid, anterior view. Click on image to enlarge
The Anterior View

To the right, you can see a diagram of the figure where the muscles overlay the skeleton on the our left and all but the the shoulder muscle have been removed on the right. This is to show how this muscle fits in with the other muscles and at the same time demonstrates the muscle's connection to the skeleton.

The large shoulder muscle is called the deltoid. As you can see the deltoid is a surface muscle that creates the major shape of the shoulder.

If you look at the right side of the diagram you can see that on the front of the body the deltoid starts at the clavicle and ends at the side of the humerus.  

The deltoid, posterior view. Click on image to enlarge

By the way, the starting location of a muscle is called the origin. This is the attachment to the part of the skeleton that the muscle does not move.

The ending location of a muscle is called the insertion. This is the attachment to the part of  the skeleton that the muscle moves.


The Posterior View

From the back, you can see that the deltoid starts at the spine (a ridge along the top) of the scapula and ends at the humerus about halfway down the shaft.

Think of the clavicle and scapula as the fixed points that give the deltoid the leverage to raise the arm.

The Shape

One common mistake is to draw the deltoid rounding above the clavicle and the scapula when the arm is resting by the side. It does not extend above those two bones. Only when the arm is raised does the body of the deltoid rise above its origin.

This is one example of why the understanding of how the muscle attaches to the skeleton helps identify the shape more accurately. 

The Action 


That leads us to our action or what the muscle does. The deltoid abducts the arm at the shoulder. This means the arm is moved away from the body when the muscle contracts.

When drawing the shoulder notice how shape of the deltoid changes, it rounds out more as it is raised. This is because the muscle fibers are contracting and condensing the shape of the muscle as it raises the arm. 
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There is more we can cover about this muscle, but this is a good start. Take a look at your shoulder, or a friend's shoulder, as it rests by the side then raise it. You will probably be able to see where the deltoid connects and notice the change in shape as you raise it.